[Bindings] Make maplike<> and setlike<> imply a readonly size attribute.

third_party/WebKit/Source/modules/peerconnection/RTCStatsReport.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef RTCStatsReport_h
 #define RTCStatsReport_h
 
 #include "bindings/core/v8/Maplike.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "platform/heap/GarbageCollected.h"
 #include "platform/wtf/text/WTFString.h"
 #include "public/platform/WebCString.h"
 #include "public/platform/WebRTCStats.h"
 
 #include <map>
 
 namespace blink {
 
 // https://w3c.github.io/webrtc-pc/#rtcstatsreport-object
 class RTCStatsReport final : public GarbageCollectedFinalized<RTCStatsReport>,
                              public ScriptWrappable,
                              public Maplike<String, v8::Local<v8::Value>> {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
   RTCStatsReport(std::unique_ptr<WebRTCStatsReport>);
 
+  size_t size() const { return report_->Size(); }
+
   // Maplike<String, v8::Local<v8::Value>>
   PairIterable<String, v8::Local<v8::Value>>::IterationSource* StartIteration(
       ScriptState*,
       ExceptionState&) override;
   bool GetMapEntry(ScriptState*,
                    const String& key,
                    v8::Local<v8::Value>&,
                    ExceptionState&) override;
 
   DEFINE_INLINE_VIRTUAL_TRACE() {}
 
  private:
   std::unique_ptr<WebRTCStatsReport> report_;
 };
 
 }  // namespace blink
 
 #endif  // RTCStatsReport_h